Mass spectral interpretation

Lee, T.A., A Beginner s Guide to Mass Spectral Interpretation, Wiley, Chichester, U.K., 1998. 

Maximum benefit from Gas Chromatography and Mass Spectrometry will be obtained if the user is aware of the information contained in the book. That is, Part I should be read to gain a practical understanding of GC/MS technology. In Part II, the reader will discover the nature of the material contained in each chapter. GC conditions for separating specific compounds are found under the appropriate chapter headings. The compounds for each GC separation are listed in order of elution, but more important, conditions that are likely to separate similar compound types are shown. Part II also contains information on derivatization, as well as on mass spectral interpretation for derivatized and underivatized compounds. Part III, combined with information from a library search, provides a list of ion masses and neutral losses for interpreting unknown compounds. The appendices in Part IV contain a wealth of information of value to the practice of GC and MS. 

The GC separations, derivatization procedures, mass spectral interpretation, structure correlations, and other information presented in this book were collected or experimentally produced over the length of a 30-year career (F.G.K.) in GC/MS. It has not been possible to reference all sources therefore, in the acknowledgments, we thank those persons whose work has significantly influenced this publication. 

II. GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types 43... 

The book is divided into four parts. Part I, The Fundamentals of GC/MS, includes practical discussions on GC/MS, interpretation of mass spectra, and quantitative GC/MS. Part II, GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types, contains chapters for a variety of compounds, such as acids, amines, and common contaminants. Also included are GC conditions, methods for derivatization, and discussions of mass spectral interpretation with examples. Part III, Ions for Determining Unknown Structures, is a correlation of observed masses and neutral losses with suggested structures as an aid to mass spectral interpretation. Part IV, Appendices, contains procedures for derivatization, tips on GC operation, troubleshooting for GC and MS, and other information which are useful to the GC/MS user. Parts I to III also contain references that either provide additional information on a subject or provide information about subjects not covered in this book. 

There are many ways to interpret mass spectra. Frequently, prior knowledge or the results from a library search dictate the method. The proceeding is a brief description of an approach to mass spectral interpretation that is especially useful when little is known about the compounds in the sample. 

The correct analysis of the homologous ion series has certain limitations. Low abundances of peaks in some series require the attention and experience of a researcher. Usually alkane series are dominated in the mass spectra of the most various compounds. Fragmentation initiated by one functional group may completely suppress or notably camouflage other reactions of polyfunctional substances. In the latter case it is useful to consider IR-spectroscopy data in mass spectral interpretation. 

There is no one-and-only approach to the wide field of mass spectrometry. At least, it can be concluded from the preceding pages that it is necessary to learn about the ways of sample introduction, generation of ions, their mass analysis and their detection as well as about registration and presentation of mass spectra. The still missing issue is not inherent to a mass spectrometer, but of key importance for the successful application of mass spectrometry. This is mass spectral interpretation. All these items are correlated to each other in many ways and contribute to what we call mass spectrometry (Fig. 1.4). 

Crosscheck proposed molecular structure and mass spectral data. This is also recommended between the single steps of mass spectral interpretation. 

The occurrence of [Mh-H] ions due to bimolecular processes between ions and their neutral molecular counterparts is called autoprotonation or self-CI. Usually, autoprotonation is an unwanted phenomenon in EI-MS. [M-i-1] ions from autoprotonation become more probable with increasing pressure and with decreasing temperature in the ion source. Furthermore, the formation of [M-i-1] ions is promoted if the analyte is of high volatility or contains acidic hydrogens. Thus, self-CI can mislead mass spectral interpretation either by leading to an overestimation of the number of carbon atoms from the C isotopic peak (Chap. 3.2.1) or by... 

Mass spectral interpretation. Box 24-1 shows the separation of enantiomers with the formula CQH4N2C16. 

In addition to software tools to help postacquisition processing, software tools to help mass spectral interpretation, particularly MS/MS, have taken new strides as well (Heinonen et al., 2008). One example of such a software tool is the MathSpec program. The details of the MathSpec approach have been explained (Sweeney, 2003). MathSpec software is used in conjunction with MS/MS spectra obtained under high-resolution conditions. The software systematically attempts to assemble possible parts (from the MS/MS fragment data) of the molecule into a rational molecule. Other examples of structure elucidation software include HighChem s Mass Frontier and ACD/Labs ACD/MS Manager (Bayliss et al., 2007). Other metabolite prediction software tools such as Meteor are also being incorporated into LC-MS software as tools to help accelerate metabolite detection and characterization (Testa et al., 2005 Ives et al., 2007). 

Bayliss, M. A., Antler, M., McGibbon, G., and Lashin, V. (2007). Rapid Metabolite Identification Using Advanced Algorithms for Mass Spectral Interpretation. In Proceedings of the 55th ASMS Conference on Mass Spectrometry and Allied Topics. ASMS, Indianapolis, IN. 

The authors wish to express their sincere thanks to the following people who have provided information for specific portions of this chapter D. E. Dorman, J. W. Paschal, and A. D. Kossoy for the NMR interpretation A. Hunt for the ultraviolet spectral interpretation B. W. Jackson for the synthetic preparation A. D. Kossoy for the degradation work J. L. Occolowitz for the mass spectral interpretation ... 

Compound identifications were made by combined gas chromatography-mass spectrometry (GC-MS) based on relative retention times and mass spectral interpretations. The instrument used was a Finnigan 5100 computerized GC-MS system equipped with a 50 m x 0.32 mm i.d. fused silica capillary coated with CP Sil 8 CB (0.25 jim film thickness). Helium was used as carrier gas and the temperature program was as follows 110°C (2 min)- 3°C/min - 320°C. 

Structural Information from Spectral Data. The kinds of information that can be derived from an unknown mass spectrum by either human or computer examination include the identities of substructural parts of the molecule (parts that both should, and should not, be present), data concerning the size of the molecule (molecular weight, elemental composition), and the reliability of each of these postulations. In our opinion, the latter is much more critical for mass-spectral interpretive algorithms than those for techniques such as NMR and IR the effect of a particular substructure on the mass spectrum is often dependent on other parts of the molecule, and a thorough understanding of these effects can only be achieved by studying the spectra of closely related molecules. 

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